Method and apparatus for drying moist exhaust air from one or more bulk material drying hoppers

ABSTRACT

The exhaust air from a bulk material drying hopper is conveyed through a chamber filled with an absorbing medium and the air is fed back to the drying hopper in a closed circuit. The chamber is regenerated at intervals in a second circuit. The air throughput through the hopper is controlled as a function of the temperature of the exhaust air.

The invention relates to a method of drying exhaust air from at leastone bulk material drying hopper, the said exhaust air being conveyedthrough at least one chamber filled with an absorbing medium and beingfed back to the drying hopper or hoppers in a closed circuit, whereinthe chambers are regenerated at intervals in a second air circuit, andto an apparatus which is particularly suited to carrying out the method.

In order to dry bulk materials of all types, in particular plasticspowders and granulates, in the bulk material drying hopper, hot drydrying air is blown into the drying hopper through the supply air pipeopening in the lower portion of the drying hopper, flows counter to thebulk material sinking through the drying hopper, heats the said bulkmaterial, absorbs moisture therefrom while cooling and leaves the dryinghopper at its upper portion through the exhaust air pipe. The dampexhaust air is then conveyed through a chamber filled with an absorbingmedium and arranged in a drier and is forced through a heating deviceinto the supply air pipe again by the blower.

If the quantity of bulk material sinking through the drying hopper fromthe top to the bottom per unit of time is reduced, the temperature ofthe exhaust air rises. This effect can lead to the exhaust air leavingthe drying hopper so hot that is enters the chamber at a relatively hightemperature. The silica gel or molecular sieve, usually employed as theabsorbing medium for the moisture, has the characteristic, however, thatits moisture absorption is sharply reduced at a higher temperature.Exhaust air at a temperature above 40° C. to 60° C. is then no longercompletely dried in the chamber, and on entering the drying hopper thesupply air already contains water vapour and thus has a poorercondensation point. In this way the drying process in the drying hoppertakes place more slowly or incompletely. In addition the temperature inthe dry air drier rises to an undesired extent, which, particularly inthe case of a closed drier housing, is harmful to the units disposedtherein. Finally, the higher return air temperature causes increasedenergy losses from the air pipes and the drier into the operating area.

The invention is thus to provide measures which prevent deleteriousheating of the dry air drier, make possible a good condensation point ofthe drying air even with a reduced granulate throughput and reduce theenergy losses occurring in the entire system from air pipes, driers andbulk material drying hopper or hoppers.

To this end the method named above is effected according to theinvention such that the amount of the air throughput through the bulkmaterial drying hopper is controlled as a function of the airtemperature. In contrast to the conventional idea that as great aquantity of drying air as possible is advantageous for the success ofthe drying, the quantity of drying air is throttled as the exhaust airtemperature rises, to the extent that the temperature of the exhaust airremains scarcely above room temperature. This results in complete dryingof the exhaust air in the chamber, avoids injurious heating of the drierand reduces the energy losses. Furthermore the invention allows the heatcontained in a chamber which has just been regenerated to be used forpreheating the supply air. Thus according to the invention a chamberwhich has just been regenerated need not be separately cooled beforebeing brought into the drying air circuit. This was always necessarybefore, since bringing an uncooled chamber into the drying air circuitat a high temperature of the exhaust air would result in a injuriouslyhigh rise in the supply air temperature.

If two or more drying hoppers are present, when they are connected inparallel it is often difficult to distribute the quantity of aircorrectly to the individual drying hoppers on account of, inter alia,the different distances of the drying hoppers from the drier and alsothe different sizes of the drying hoppers. This distribution is alsomade difficult by the fact that widely differing materials, which inaddition present an entirely different resistance to the air flowingthrough the drying hopper, has to be dried in the individual dryinghoppers.

Even if it is possible to distribute the entire quantity of air whichmay be delivered by a drier correctly to the individual drying hoppers,for example by manual adjustment of throttle valves provided in thesupply pipes to the individual drying hoppers, the equilibrium onceattained is considerably impaired by the fact that unexpectedly moregranulate mass is removed from a drying hopper than was intended. If,however, more or less drying air than is required by the drying processtaking place in a drying hopper is supplied to the said drying hopper,the phenomena described above occur.

In a further development of the invention, a deleterious heating of thedrying air drier is prevented and a good condensation point of thedrying air is made possible even with a reduced granulate throughputthrough one of the drying hoppers, when the quantity of the drying airflowing through each bulk material drying hopper is regulated as afunction of its emergent temperature from the respective drying hopper.In this way even a multi-drying-hopper system may be combined with thedrier without energy losses or injurious heating of the said drier.

The invention also provides an apparatus for drying damp exhaust airsupplied from a bulk material drying hopper by way of an exhaust airpipe, comprising at least one chamber filled with an absorbing medium, aregeneration device for the chamber and a blower, the pressure side ofwhich is connected to a supply air pipe leading into the drying hopperand provided with a heating device, a temperature sensor influencing acontrol device being arranged in the exhaust air pipe. This apparatusallows the method according to the invention to be carried out in aparticularly simple manner. The control device can control either athrottle arranged in the supply air pipe, a by-pass pipe connecting thepressure side to the inlet side of the blower or the drive of the blowerin the sense of an alteration of the air throughput controlled by theexhaust air temperature.

If, in a further development of the invention, the control device isconnected to a device determining the regeneration frequency of thechambers in the regeneration device, the regeneration frequency of thechambers may be reduced in the same ratio as the reduction of thequantity of air flowing through the drying hopper. This represents amajor saving in energy, since in the case of dry air driers theregeneration process has a relatively high energy requirement. By thisadditional use of the control device the use of expensive moisturesensors, by means of which the regeneration frequency is controlled, isdispensed with. Alternatively the further development of the inventionprovides for a manually operable control device for controlling the airthroughput through the supply air pipe. In this way the dry air drieraccording to the invention becomes extremely economical and efficient,as the amount of the dry air throughput may be adjusted according to thequantity of the bulk material present which is to be dried and itsmoisture content with reference to predetermined guidelines. Anadditional energy saving is obtained by combining the volume control ofthe dry air with a manually adjustable regeneration frequency.

Finally, the invention may also be realized by connecting the controldevice to a device determining the transition rate from the regenerationphase to the drying phase on the regeneration device. If the quantity ofdry air is throttled, a newly regenerated uncooled chamber may bebrought into the dry air circuit substantially more slowly than in thecase of a higher dry air throughput. The control device may also containa delaying element which effects a reduction of the dry air throughputthrough the supply air pipe with a delay when bringing a newlyregenerated, hot chamber into the drying phase.

A particularly advantageous further development of the invention, in thecase of the apparatus, provides that when a plurality of bulk materialdrying hoppers are arranged in parallel a temperature sensor influencingthe control device is disposed in each air pipe of a bulk materialdrying hopper, it also being particularly recommended that a temperaturesensor controlling a motor-driven throttle valve by way of a separatecontrol line should be arranged in the air pipe leading to each dryinghopper.

In addition, advantageous further developments of the invention are setout in the sub-claims.

The invention is explained in detail below with reference to theembodiments illustrated in the accompanying drawings, in which

FIG. 1 is a diagrammatic cross-sectional view of a bulk material dryinghopper with a drier, and

FIG. 2 is a plan view of an arrangement of a plurality of bulk materialdrying hoppers connected in parallel to a drier in accordance with FIG.1.

In a housing 1 of a dry air drier five chambers 2 are arranged on arotating frame which is slowly rotated by a drive motor 3 about its axis4. The chambers are filled with an absorbing medium 5. A blower 6 sucksair via a filter 7 from an exhaust air pipe 8 which is connected to abulk material drying hopper 9, and forces it via a distributing duct 10through four of the five chambers 2 filled with the absorbing medium 5.The drying air flows from these chambers via a collecting duct 11through a heater 12 which is adjusted to the desired preheatingtemperature of the emergent drying air by a thermostat 13, and via asupply air pipe 14 to the drying hopper 9. The supply air pipe 14 opensinto an air distributor 15 which extends deep into the drying hopper 9.The dry preheated drying air now slowly rises up through the plasticsgranulate 16, absorbs the moisture of the latter and gives off part ofits heat. The cycle then begins again via the exhaust air pipe 8 and thefilter 7.

Of the five chambers 2 the chamber 2a shown on the left in the drawingis connected in a regeneration circuit which is completely separate fromthe circuit of the drying air and in which a further filter 17, a blower18 and a heater 19 are arranged. The blower 18 draws in outside air viathe filter 17 and forces it via the heater 19 through the absorbingmedium 5a saturated with moisture into the chamber 2a. The heated airabsorbs the moisture of the absorbing medium 5a in the chamber 2a andescapes into the atmosphere via the tube 20.

As soon as the regeneration process of the absorbing medium, i.e. theremoval of the moisture, is completed, the newly regenerated chamber 2ais rotated into the dry air circuit by the motor 3 and is available fordrying air, while an adjacent chamber heavily saturated with moisture isrotated into the regeneration circuit. The quantity of dried granulatedmaterial 16 removed at the removal box 21 of the drying hopper 9 iscontinuously replaced by damp granulated material from the conveyingmeans 22.

The temperature-sensitive element of a temperature sensor 30 extendsinto the exhaust air pipe 8 and detects the temperature of the exhaustair from the drying hopper 9. The temperature sensor 30 is connected toan input of a control device 31 by way of an output line 35 andtransmits a signal corresponding to the detected temperature of theexhaust air to the control device 31. The control device 31 compares thedetected temperature of the exhaust air with a nominal value whichcorresponds, for example, to a temperature slightly above roomtemperature. If the temperature registered by the temperature sensor 30is above the reference value, the control device 31 acts upon the driveof the blower 6 by way of a line 37 in such a way that the blower 6forces a reduced amount of air through the chambers and the supply airpipe 14 into the drying hopper 9. This throttling of the amount ofsupply air fed in may be effected in a manner not shown either byreducing the rotational speed of the blower 6 or by opening a feedbackpipe 51 from the delivery side of the blower 6 to its suction side andallowing an appropriate proportion of the quantity of dry air to flowback.

In addition a branch line 52 leads from the control line 38 to athrottle 50 in the supply air pipe 40 so that a control signal emittedby the control device 31 can control the throttle 50 as a function ofthe exhaust air temperature detected by the temperature sensor 30.

A branch line 38 and 39 from the line 37 leads to a device 32 whichdetermines the regeneration frequency of the chambers by accelerating orretarding the motor 3 of the rotating means. If the blower 6 iscontrolled by the control device so as to reduce the amount of airsupplied, therefore, the regeneration frequency is reduced by the device32 in order to utilize to the full the moisture-absorption capacity ofthe absorbing medium 5 in the chambers 2 and thus to save energy.

In addition, the drive motor 3 is controlled by a device 34 which allowsthe transition rate from the regeneration phase to the drying phase of afully regenerated chamber 2a to be prolonged if the quantity of supplyair moved by the blower 6 is reduced. To this end the branch line 38leads from the control device 31 via a line 41 to the device 34.

The line 39, which leads from the branch line 38 to the device 32, mayalso be acted upon by a signal from a control device 33 via the line 40when one of the manually operable adjustment knobs 42 is set so as toprolong the regeneration cycle period in accordance with a predeterminedtabular value. The tabular value takes into consideration the nature ofthe plastics granulated material contained in the drying hopper 9, thehourly throughput through the drying hopper and its moisture content. Afurther adjustment knob 43 on the control device 33, which allows thedrive of the blower 6 to be directly acted upon by a control signal viaa line 44, allows the quantity of supply air conveyed by the blower 6 tobe controlled independently of the regeneration cycle period. Inaddition, the control device 31 contains a delay element, not shown,which allows a control signal increasing the amount of air conveyedthrough the blower 6 to reach the line 37 with a delay when a newlyregenerated hot chamber 2a is rotated into the drying phase by the motor3.

The invention may also be applied in the same way with a closed housing1 for the dry air drier. In the case of a closed dry air drier theventilation grille 23 and the connecting line 25 to the filter aredispensed with. Instead, the quantity of air required for theregeneration cycle is sucked in by the blower 18 under the cover of thehousing 1. In addition, suction lines are run from the filter 17 to themotor 3 (and) to the blower 6, which are cooled by the outside airsucked in.

A position sensor 55, the sensor 56 of which is contacted by aregenerated chamber 2a during its advance through the regenerationphase, is also secured in the housing 1 in the path of the chamber 2a.The position sensor 55 controls, in a manner not shown, the drive motor3 for the rotating frame with the chambers 2, 2a and the other chambersnot provided with reference numerals. In this way, before bringing aregenerated chamber into the drying phase, the rotating frame isdetained until the chamber which has remained in the drying phase thelongest is sufficiently saturated. The frequency with which the airthroughput through the drying hopper would be throttled by the sensor 30and the control device 31 may be taken as a criterion for the moistureabsorbed in a chamber.

It is also within the scope of the invention to arrange the temperaturesensor 30 in the supply air pipe 14, preferably downstream of thethrottle 50, in order to obtain an indirect reading of the exhaust airtemperature in this way.

According to FIG. 2 bulk material drying hoppers 100, 110, 120 withdifferent filling quantities of from 1,000 liters to 200 liters areconnected in parallel to a drier 80 of the type illustrated in FIG. 1via a supply air pipe 160 and an exhaust air pipe 170. From the supplyair pipe 160 a portion 106 leads to the drying hopper 100, a supply airbranch pipe 116 leads to the drying hopper 110 and a supply air branchpipe 126 leads to the drying hopper 120. From the drying hopper 100 aportion 107 leads to the exhaust air pipe 170, and from the dryinghopper 110 a branch exhaust air pipe 117 and from the drying hopper(120) a branch exhaust air pipe 127 respectively lead to the exhaust airpipe 170.

Directly in front of the inlet to each of the drying hoppers 100, 110,120 a throttle valve 20', 21', 22' is disposed in the pipes 106, 116,126 respectively, the position of the said throttle valves in thecross-section of the associated supply air pipe being controlled by asetting motor 20", 21", 22" respectively. From each of the settingmotors 20", 21", 22" respectively a separate control line 101, 111, 121leads to a temperature sensor 100', 110', 120' respectively, which isarranged in the other air pipe 107, 117, 127 leading to the associatedbulk material drying hopper 100, 110, 120 respectively. Each of thetemperature sensors 100', 110', 120' delivers a setting signal via theassociated control line 101, 111, 121 to the setting motor 20", 21", 22"respectively connected to the associated control line, and this signalcauses the said setting motor to move the throttle valve 20', 21', 22'into a corresponding position in accordance with a characteristicmagnitude of the control signal. If, for example, the exhaust air fromthe bulk material drying hopper 100 is too hot, the throttle valve 20'is closed again by the setting motor 20".

It is also within the scope of the invention for each of the throttlevalves 20', 21', 22' also to be manually adjustable to a selectedinitial position. Finally, the invention may also be realized by thecontrol lines 101, 111, 121 being led separately to a microprocessor,not shown, which is disposed for example in the control device 31 of thedrier 80 and which, where necessary while evaluating further measurementvalues taken from the bulk material drying hoppers 100, 110, 120, allowsthe control signals delivered by the temperature sensors 100', 110',120' to produce control signals individually to the respective settingmotors 20", 21", 22" via control lines not shown. In this way a centralcontrol of the volume of air throughput both through each of and throughall the bulk material drying hoppers 100, 110, 120 is possible, so thatthis interpretation of the invention produces a particularly sensitiveadjustment of the air throughput to the respective operating phases andconditions of the individual bulk material drying hoppers 100, 110, 120.In addition, it is within the scope of the invention to arrange thetemperature sensor in the exhaust air branch pipe and themotor-controlled throttle valve in the supply air branch pipe on the onehand and the temperature sensor in the supply air branch pipe and thethrottle valve in the exhaust air branch pipe on the other.

In the above description and in the claims the term "air" also embracesordinary gases, such as nitrogen, the use of which is recommended inmany cases according to the granulated material to be dried and/or thedry mass in the drier.

I claim:
 1. In a method of drying exhaust air from one or more bulkmaterial drying hoppers, comprising the steps of conveying exhaust airfrom a bulk material drying hopper through at least one chamber filledwith an absorbing medium and feeding the air back to the drying hopperin a closed circuit, the chamber being regenerated at intervals in asecond air circuit, the improvement comprising the step of controllingthe amount of the air throughput through the bulk material drying hopperas a function of the temperature of the exhaust air from the dryinghopper.
 2. The method of claim 1, in which the volume of the airthroughput is controlled by altering the rotational speed of a blowerconnected to the chamber.
 3. The method of claim 1, in which the volumeof the air throughput is controlled by a throttle arranged in an airpipe for the drying hopper.
 4. The method of claim 1, in which thevolume of the air throughput is controlled by opening and closing aby-pass pipe of a blower connected to the chamber.
 5. The method ofclaim 1, in which at least two bulk material drying hoppers are inparallel and the quantity of the drying air flowing through each bulkmaterial drying hopper is regulated as a function of its emergenttemperature from the respective bulk material drying hopper.
 6. Themethod of claim 1, in which a newly regenerated chamber in the uncooledstate is transferred to the drying phase.
 7. The method of claim 6, inwhich directly before bringing a hot regenerated chamber into the dryingcircuit the quantity of air is throttled in accordance with thetemperature of the exhaust air.
 8. In apparatus for drying moist exhaustair supplied from one or more bulk material drying hoppers by way of anexhaust air pipe, comprising at least one chamber filled with anabsorbing medium, a regeneration device for the chamber, and a blowerwhose pressure side is connected to a supply air pipe leading into thedrying hopper and provided with a heating device, the improvementcomprising temperature sensing means disposed in an exhaust air pipeleading from the drier and a control means operably connected to thetemperature sensing means for controlling the amount of air supplied tothe drying hoppers as a function of the temperature of exhaust air insaid exhaust pipe.
 9. The improvement of claim 8, in which the controlmeans is operably connected to means for driving the blower forcontrolling blower speed as a function of said exhaust air temperature.10. The improvement of claim 8, in which said control means is operablyconnected to means for determining the regeneration frequency of thechamber.
 11. The improvement of claim 8, including a manually operablemeans for controlling the air throughput through the air pipe.
 12. Theimprovement of claim 8, including a second manually operable controlmeans for controlling the regeneration frequency of the chamber.
 13. Theimprovement of claim 8, in which the control means is connected to ameans for determining the transfer speed of a newly regenerated hotchamber into the drying circuit.
 14. The improvement of claim 8, inwhich the control means includes a delaying element which effects areduction of the air throughput through the air pipe with a delay whenbringing a newly regenerated, hot chamber into the drying phase.
 15. Theimprovement of claim 13 or 14, including a rotating frame comprising aplurality of the chambers, each of which is selectively connected tosaid drying hoppers for drying air and disconnected therefrom forregeneration, position sensor means operably connected to said rotatingframe, and a delaying element controlled by the position sensor, fortemporarily reducing the drive speed of the rotating frame.
 16. Theimprovement of claim 8, including a closed housing accommodating thecontrol device and the blower.
 17. The improvement of claim 8, includinga plurality of bulk material drying hoppers arranged in parallel, atemperature means influencing the control means being arranged in eachair pipe of a bulk material drying hopper.
 18. The improvement of claim8, including a plurality of bulk material drying hoppers arranged inparallel, the temperature sensing means operably connected to andcontrolling a motor-controlled throttle valve by way of a separatecontrol line disposed in the exhaust air pipe of each bulk materialdrying hopper.
 19. In apparatus for drying moist exhaust air suppliedfrom one or more bulk material drying hoppers by way of an exhaust airpipe, and for returning dried air to said hoppers, wherein saidapparatus includes at least one air drying chamber filled with amoisture absorbing medium, a regeneration apparatus for the chamber anda blower means for moving air through the hopper and drying chamber, theimprovement comprising:temperature sensing means disposed in saidexhaust air pipe for sensing temperature of said moist exhaust air, andcontrol means operably connected to said temperature sensing means forvarying the amount of dried air supplied from said drying chamber tosaid hopper in response to changes in the sensed temperature of saidmoist exhaust air.
 20. The improvement of claim 19, wherein said controlmeans is operably connected to said blower means for varying the amountof air supplied to said hopper.
 21. The improvement of claim 19, furtherincluding a plurality of drying chambers and second control means forcontrolling the regeneration frequency of said chambers as a function ofthe change in temperature of said exhaust air.
 22. The improvement ofclaim 19, further including manual control means for controlling theamount of air supplied to said hopper.
 23. The improvement of claim 19,further including a plurality of drying chambers and a manual controlmeans for controlling the regeneration frequency of each chamber. 24.The improvement of claim 19, further including a plurality of dryingchambers, means for selectively removing a chamber from a dry air supplycircuit to said hopper for regeneration and means for controlling thetransfer speed of a regenerated drying chamber into the supply circuit.25. The improvement of claim 19, including a plurality of dryingchambers, means for selectively removing a drying chamber from an airsupply circuit to said hopper for regeneration and for reintroducing theregenerated chamber to said circuit and delay means for reducing theamount of dry air supplied to said hopper when reintroducing a newlyregenerated chamber into said supply circuit.
 26. The improvement ofclaim 19, including a plurality of drying chambers mounted on a rotatingframe for rotation out of a dry air supply circuit for said hopper forregeneration and reintroduction thereto after regeneration, a chamberposition sensor and delay means operably connected to said rotatingframe for varying the speed of said frame and the reintroduction ofchambers to said supply circuit.
 27. The improvement of claim 19,including a plurality of bulk material drying hoppers disposed in aparallel air circuit, each hopper having an exhaust passage and hopperexhaust air temperature sensing means disposed in each exhaust passage.28. The improvement of claim 27, including a dry air supply ductsupplying each hopper, a throttle valve means disposed in each supplyduct and said valve means for each hopper operably connected to thetemperature sensing means of each hopper for varying the amount ofdrying air supplied to its respective hopper in response to variationsin temperature of the exhaust air of the respective hopper.